At the present time, virtually all printed copy is produced through the use of three basic types of printing plates. One type is a relief plate which prints from a raised surface. Another type is an intaglio plate which prints from a depressed surface. The third type is a lithographic plate which prints from a substantially flat surface which is neither appreciably raised above nor appreciably depressed below the adjacent and surrounding non-printing areas. Printing is occasioned by an ink's respective affinity and/or aversion to areas of different chemical properties. Lithographic printing plates are commonly processed to have water-repellent (hydrophobic), oil-receptive (oleophilic) image areas and water-receptive (hydrophilic) non-image areas.
Prior to processing for use, conventional lithographic plates will typically have a hydrophobic, photoreactive polymeric image layer (i.e., photoresist) coated or otherwise deposited atop a hydrophilic substrate.
In preparing a conventional lithographic plate for use on a printing press, the plate is first exposed to actinic radiation. Specific chemical reactions are caused to occur in the plate's photoresist by exposure to actinic radiation. Such photoinduced chemical reactions may either reduce or enhance the solubility of the photoresist, depending on whether the resist is negative-working or positive-working. In negative-working plates, exposure to actinic radiation will generally cause a "hardening" of the photoresist. In positive-working plates, exposure to actinic radiation will generally cause a softening or solubilization of the photoresist.
After photoexposure, a wet development step is normally conducted. The objective of such wet development is to remove those areas of the photoresist which have undergone photoinduced chemical change or those which have not been photoexposed. Solvation under conventional development techniques will typically involve treating the exposed plate with organic solvents in a developing bath. For negative-working resists, the solvent will swell and dissolve the unexposed portions of the resist. The solvent should not swell the exposed portions or distortion of the developed image may result. For positive-working resists, the response of the unexposed and exposed coatings are reversed, but the same general principles apply.
As a result of the preferential solvation and washing away of portions of the photoresist, corresponding portions of the underlying hydrophilic substrate are uncovered. For negative-working plates, the aforementioned hydrophobic image areas correspond to the portions of the photoresist remaining after solvation and washing. The aforementioned hydrophilic non-image areas correspond to uncovered portions of the substrate. The image and non-image areas thus differentiated, the processed plate may then be mounted onto a printing press and run.
Encumbered by required wet development, the processing of conventional lithographic plates prior to their use on a printing press is both time and labor consuming and involves considerable use of organic chemicals. It will be appreciated that there is a considerable desire for means that would satisfactorily eliminate or reduce conventional lithography's long-felt dependency upon the conduct of wet development and thereby permit use of lithographic plates on a printing press immediately after exposure without required post-exposure prepress processing.
In the past, dry developable lithographic printing plates have been suggested which enable the wet processing steps of lithographic printing plates after exposure to be omitted and printing to be conducted by directly mounting the exposed plates on a printing press. Among printing plates that may be characterized as on-press developable (or related thereto) are: e.g., U.S. Pat. No. 4,273,851, issued to Muzyczko et al. on Jun. 16, 1981; U.S. Pat. No. 4,879,201, issued to Hasegawa on Nov. 7, 1989; U.S. Pat. No. 4,916,041, issued to Hasegawa et al. on Apr. 10, 1990; U.S. Pat. No. 4,999,273, issued to Hasegawa on Mar. 12, 1991; and U.S. Pat. No. 5,258,263, issued to Z. K. Cheema, A. C. Giudice, E. L. Langlais, and C. F. St. Jacques on Nov. 2, 1993.
Despite the methodologies and approaches embodied in the aforementioned patents, there is a continuing need for a lithographic printing plate that can be readily developed on a printing press and that produces a plate having durable image areas needed for good run length. Applications for such on-press developable printing plates have been filed.
U.S. patent application Ser. Nos. 08/147,045 and 08/146,711 (now abandoned), filed by W. C. Schwarzel, F. R. Kearney, M. J. Fitzgerald, and R. C. Liang on Nov. 1, 1993, describe a photoreactive polymeric binder that may be used to enhance photospeed in either conventional plates or on-press developable lithographic printing plates. Briefly, a polymer of m-isopropenyl-.alpha.,.alpha.-dimethylbenzyl isocyanate is derivatized for vinyl group reactivity by reacting the isocyanate groups thereof with a hydroxyalkyl acrylate, such as 4-hydroxybutyl acrylate. The resulting photopolymeric binder provides higher photospeed than compositions containing non-reactive binders typically utilized in the production of printing plates. Lithographic printing plates utilizing the photoreactive polymeric binder have good durability (as manifested by good run-length) and can be developed using relatively weak developers. As to the preparation of the photoreactive binders, the applications describe a method of copolymerizing m-isopropenyl-.alpha.,.alpha.-dimethylbenzyl isocyanate through complexation with an electron-deficient monomer (e.g., maleic anhydride) to accelerate free radical copolymerization with other monomers. The maleic anhydride accelerated process is kinetically more efficient and provides greater monomer-to-polymer conversion. Use of the resulting product in the photoresist of a lithographic printing plate improves its adhesion. The disclosures of commonly assigned U.S. patent application Ser. Nos. 08/147,045 and 08/146,711 are hereby incorporated by reference. Reference is also made to U.S. patent application Ser. No. 08/429,804, commonly assigned and filed on Apr. 27, 1995.
U.S. patent application Ser. No. 08/147,044, filed by F. R. Kearney, J. M. Hardin, M. J. Fitzgerald, and R. C. Liang on Nov. 1, 1993, now abandoned, describes the use of plasticizers, surfactants and lithium salts as development aids for negative-working, on-press developable lithographic printing plates. Briefly, plasticizers, which are dispersible or soluble in press fountain solutions and soluble in acrylic monomers and oligomers, are incorporated into a photoresist. Such plasticizers make the photoresist more permeable to fountain solution prior to crosslinking, while being easily extracted with ink and fountain solution after crosslinking. The surfactants facilitate the dispersion of hydrophobic imaging compositions in the fountain solution and reduce scumming. Further, lithium salts may also be incorporated into the photoresist to disrupt hydrogen bonding of, for example, urethane acrylate polymers which tend to associate by hydrogen bonding, thus enhancing developability. The disclosure of commonly assigned U.S. patent application Ser. No. 08/147,044 is hereby incorporated by reference.
U.S. patent application Ser. No. 08/146,479, filed by L. C. Wan, A. C. Giudice, W. C. Schwarzel, C. M. Cheng, and R. C. Liang on Nov. 1, 1993, now abandoned, describes the use of rubbers and surfactants to enhance the durability of on-press developable printing plates. The rubbers are preferably incorporated into a photoresist as discrete rubber particles. To ensure a uniform and stable dispersion, the rubber components are suspended in the photoresist preferably by means of surfactants having HLBs approximately between 7.0 and 18.0. The disclosure of commonly assigned U.S. patent application Ser. No. 08/146,479, is hereby incorporated by reference.
While the practice of the subject matter set forth in the aforementioned applications can produce suitable "on-press" developable printing plates, the subject matter is desirably combined with that of U.S. patent application Ser. No. 08/146,710, filed by L. C. Wan, A. C. Giudice, J. M. Hardin, C. M. Cheng, and R. C. Liang on Nov. 1, 1993, (commonly assigned and incorporated herein by reference). U.S. patent application Ser. No. 08/146,710 describes a lithographic printing plate for use on a printing press, with minimal or no additional required processing after exposure to actinic radiation. Plate embodiments comprise a printing plate substrate, a polymeric resist layer capable of imagewise photodegradation or photohardening, and a plurality of microencapsulated developers capable of blanket-wise promoting the washing out of either exposed or unexposed areas of the polymeric resist. The microencapsulated developers may be integrated into the polymeric resist layer, or may form a separate layer deposited atop the polymeric resist layer, or--in certain other embodiments--may be coated onto a separate substrate capable of being brought into face-to-face contact with the resist layer.
While the on-press plate development strategies mentioned in U.S. patent application Ser. Nos. 08/146,710, 08/146,479, 08/147,044, 08/147,045, and 08/146,711 provide good results, satisfaction of requirements particular to certain applications (e.g., substantial reduction of "tackiness" of the plate and substantial reduction of curl upon the mounting thereof) effects consideration of means to maintain or further enhance photoreactivity (e.g., photospeed), such photoreactivity being potentially compromised by said strategies. In this regard, a correlation is drawn between photoreactivity and the mechanisms underlying the generation of photoactivated (or photoexcited) reactants in a printing plate. Elevated to an excited state by exposure to actinic radiation, such photoexcited species (e.g., initiator, sensitizers, coinitiators)--the presence thereof being central to the conduct of a latent image-forming photoreaction--are sensitive to oxygen. Printing plates based on free-radical initiated photocuring mechanisms, for example, are known to be susceptible to quenching by triplet oxygen. The premature return of the photoinitiator or sensitizer from excited state to energy ground state due to undesired quenching by ambient oxygen may preclude the required energy or electron transfer to effect a desirable rate and/or degree of photocuring. The photogenerated radicals also react with the oxygen and form peroxy radicals which are relatively non-reactive in the photoreaction.
A method useful for preventing oxygen quenching of radiation-generated free-radicals would be to overcoat the base coating of a printing plate with a water-soluble polymeric resin. See e.g., U.S. Pat. Nos. 5,340,681; 5,286,594; 5,120,772; 4,999,271; 4,927,737; 4,780,392; 4,707,437; and 4,652,604. In conventional configurations, the resins are transparent, film-forming polymers. These polymers are typically inert, capable of acting act as an oxygen barrier, and soluble in water or mixtures of water and solvents.
Conventional overcoats are removed off-press, typically during bath development. Since the use of "strong" solvents and vigorous scrubbing are liberally permissible under standard bath development regimens, conventional overcoats are generally tough and resilient.
In view of its ability to reduce tackiness and improve plate photospeed by preventing oxygen quenching, it becomes desirable to provide printing plates--especially the highly fountain swellable or permeable (oftentimes "tacky") on-press developable plates discussed in U.S. patent application Ser. Nos. 08/146,479; 08/147,044; 08/147,045; and 08/146,711--with an overcoat. Such overcoat would be designed to be highly soluble in printing press fountain or ink solution, and accordingly, on-press removable. However, while advantage in terms of reduced tackiness and good photospeed is accomplished by the use of an overcoat, poor shadow resolution and low contrast are sometimes observed. Poor ink receptability (cf., "ink blinding") on initial press prints is also sometimes observed.
These shortcomings also manifest to a degree in certain on-press printing plates utilizing microencapsulated developers. See, U.S. patent application Ser. No. 08/146,710. For example, in an "in-situ" printing plate system, microencapsulated developers are applied as an aqueous dispersion over a photosensitive imaging layer. This aqueous microcapsule layer in effect functions as an oxygen barrier layer. After exposure of the photosensitive layer through the microcapsule layer, the plate is run on the printing press. As with overcoated, fountain swellable or permeable on-press plates, loss of resolution and ink-receptability are sometimes observed.
It is advanced that loss of resolution and ink receptability can be attributed to (1) intermixing of an overcoat's hydrophilic components with surface of an underlying imaging layer where the components are either physically trapped or chemically bonded to the imaging layer's polymer gel by participating in the photoinduced free radical polymerization and grafting processes occurring in exposed areas, and (2) reduced effective oxygen concentration in non-image areas during exposure. Ultimately, however, these causative factors may be traced to the incidence of undesirable photoreactions in image and/or non-image areas on the printing plate surface. Thus, to broadly control the aforediscussed problems, means are needed for deactivating (or otherwise regulating) photoreactions at the surface of a polymeric resist without interfering with photoreactions in remaining areas. In on-press developable printing plates utilizing a non-tacky overcoat, means are also needed for enhancing the on-press removability of the overcoat.